Process isolation diaphragm assembly for metal process seal

ABSTRACT

A process fluid pressure transmitter includes a sensor body having a pressure sensor and electronics coupled to the pressure sensor to obtain an indication of pressure from the pressure sensor. At least one process fluid isolation assembly is fluidically coupled to the pressure sensor and is configured to receive a process fluid. The process fluid isolation assembly includes an isolation diaphragm welded to a weld ring. The weld ring has a sealing surface on a first side adapted for contact with a metal sealing ring and a weld portion welded to the sensor body on a second side. The sealing surface and the weld are substantially aligned with one another.

BACKGROUND

Process devices, such as process fluid pressure transmitters, generallysense pressure using a pressure sensor coupled to at least one isolationdiaphragm. The isolation diaphragm isolates the pressure sensor fromprocess fluids that are being sensed. Process fluids, which can behighly corrosive, are thus kept isolated from the pressure sensor toavoid corrosion or damage to the pressure sensor. Pressure istransferred from the isolation diaphragm to the pressure sensor using asubstantially incompressible isolation fluid in a passageway thatfluidically couples the isolation diaphragm to a sensing diaphragm ofthe pressure sensor. The sensing diaphragm deflects in response to theapplied pressure, and the deflection causes a change in an electricalparameter, such as capacitance, of a structure attached to or associatedwith the sensing diaphragm

The process fluid pressure transmitter is generally coupled to theprocess using a manifold or other suitable structure. The process fluidpressure transmitter is sealed to the manifold to ensure that processfluid does not leak. In a typical process fluid pressure transmitter,the sealing surface of the process fluid pressure transmitter contacts anon-metallic seal or other suitable structure. Welds for attaching theisolation diaphragm are sometimes located on the same surface that uponwhich the seal is seated. However, the non-metallic seal is compliantenough to tolerate variations in the surface of the weld and yet stilleffectively seal to the welded surface

Some non-metallic seals can retain pressures over 6000 psi. However, forhigher temperature applications where the normal working pressure isover 6000 psi metal seals can provide some advantages. When using metalseals, for high pressure applications, the welds for attaching theisolation diaphragm cannot typically be located on the sealing surfacedue to specific surface finish requirements of the metal seals.

Providing a process isolation diaphragm assembly that retains all of theadvantages of metal seals, with fewer drawbacks, would represent animprovement to process isolation diaphragm assemblies for high pressureprocess fluid applications.

SUMMARY

A process fluid pressure transmitter includes a sensor body having apressure sensor and electronics coupled to the pressure sensor to obtainan indication of pressure from the pressure sensor. At least one processfluid isolation assembly is fluidically coupled to the pressure sensorand is configured to receive a process fluid. The process fluidisolation assembly includes an isolation diaphragm welded to a weldring. The weld ring has a sealing surface on a first side adapted forcontact with a metal sealing ring and a weld portion welded to thesensor body on a second side. The sealing surface and the weld aresubstantially aligned with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a process fluid differential pressuretransmitter with which embodiments of the present invention areparticularly useful.

FIG. 2 is a diagrammatic view of a non-metallic seal used in conjunctionwith a known process isolation diaphragm assembly.

FIG. 3 is a diagrammatic view of a process isolation diaphragm assemblyutilizing a metal seal.

FIG. 4 is a diagrammatic view of a process isolation diaphragm assemblyutilizing a metal seal in accordance with an embodiment of the presentinvention.

FIG. 5 is a flow diagram of a method of assembling a process fluidpressure transmitter assembly in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an exemplary process fluid pressure transmitter 10 withwhich embodiments of the present invention are particularly useful.Transmitter 10 includes transmitter body 12, coupling flange or manifold13 and sensor body 14. Although embodiments of the present inventionwill be described with respect to a coplanar flange, embodiments of thepresent invention may be practiced on any kind of flange, manifold, orother coupling adapter that receives a process fluid.

Sensor body 14 includes pressure sensor 16, and transmitter body 12includes transmitter circuitry 20. Sensor circuitry 18 is coupled totransmitter circuitry 20 through communication bus 22. Transmittercircuitry 20 sends information related to pressure of the process fluidover a process communication link such as a two wire process controlloop (or circuit).

In some circumstances, pressure sensor 16 can measure a difference inpressure between pressure P1 in passageway 24 and pressure P2 inpassageway 26 of flange 13. Pressure P1 is coupled to sensor 16 throughpassageway 32. Pressure P2 is coupled to sensor 16 through passageway34. Passageway 32 extends through coupling 36 and tube 40. Passageway 34extends through coupling 38 and tube 42. Passageways 32 and 34 arefilled with a relatively incompressible fluid such as silicone oil.

Passageway 24 is positioned adjacent opening 28 in sensor body 14 andpassageway 26 is positioned adjacent opening 30 in sensor body 14.Diaphragm 46 is positioned in opening 28 and is coupled to sensor body14 adjacent to passageway 24. Passageway 32 extends through coupling 36and sensor body 14 to diaphragm 46. Diaphragm 50 is coupled to sensorbody 14 adjacent passageway 26. Passageway 34 extends through coupling38 and sensor body 14 to diaphragm 50.

In operation, flange 13 presses against seals 48 and 52 when transmitter10 is bolted to flange 13. Seal 48 is seated on sensor body 14 adjacentto opening 24 and diaphragm 46, and prevents process fluid leakage frompassageway 24 and opening 28 past flange 13 to the outside environment.Similarly, seal 52 is coupled to sensor body 14 adjacent to opening 26and diaphragm 50, and prevents process fluid leakage from passageway 26and opening 30 past flange 13 to the outside environment.

FIG. 2 is a diagrammatic view of a non-metallic seal used in conjunctionwith a known process isolation diaphragm assembly. Isolation diaphragm58 is welded about its periphery to weld ring 60. Weld ring 60 is thenwelded to sensor body 14 via module weld 62. Once weld ring 60 is weldedto sensor body 14, the fill fluid may be introduced into the systemwhich passes through passageway 32 and fills chamber 64. Accordingly, asprocess fluid acts on isolation diaphragm 58, the movement of isolationdiaphragm 58 generates fill fluid movement in passageway 32 whichmovement conveys pressure to differential pressure sensor 16. When thepressure transmitter is to be mounted to flange 13, a seal, such asnon-metallic seal 66 is used, which engages groove 68 of weld ring 60.Non-metallic seals work very well for applications with low to moderateprocess fluid pressures, as well as applications at low to moderateprocess fluid temperatures. As shown in FIG. 2, process isolationdiaphragm 58 is attached to sensor body 14 by welding through asecondary support piece (weld ring 60) with diaphragm 58 sandwichedtherebetween. The weld is located at the bottom of groove 68 that isalso used for seating non-metallic seal 66. Non-metallic seal 66 is ableto seal over the surface of module weld 62, but it is limited to beingable to seal to about 6000 psi and its sealing ability decreases withextreme hot and/or cold temperature exposure.

Metallic seals have very specific requirements relative to the surfacesupon which they may seat against. The exposed surface of a weld that hasgone through a weld-ring does not meet such surface requirements.

One way to overcome this surface requirement is to move the sealingsurface from directly above or adjacent the weld to an offset distance,such as shown in FIG. 3. FIG. 3 illustrates a metallic seal (C-ring 72)coupling between flange 13 and weld ring 70. The weld between weld ring70 and sensor body 14 is shown offset from metal seal 72. While theconfiguration shown in FIG. 3 ensures that the surface requirements formetal seal 72 can be achieved, it is apparent that the metal seal isacting against a portion of weld ring 70 that imparts a moment or torquewithin weld ring 70. Accordingly, the design shown in FIG. 3 may besusceptible to problems that reduce its fatigue life for processpressures above 6000 psi. Additionally, the radial offset between metalseal 72 and module weld 74 can make the assembly sensitive to pre-loadinduced by metal seal 72, which can negatively affect transmitterperformance.

FIG. 4 is a diagrammatic cross sectional view of a process isolatingdiaphragm assembly for a metal process seal in accordance with anembodiment of the present invention. Isolation diaphragm 58 is welded toweld ring 80 at weld 82. Additionally, diaphragm spacer 84 is set ormounted on the opposite side of diaphragm 58 from weld 82. Spacer 84functions to allow for a more reliable weld of isolation diaphragm 58 toweld ring 80. As shown in FIG. 4, seal 72 does not seat in any groove ofweld ring 80. Instead, weld ring 80 is substantially a solid rectanglein cross-section. Moreover, the portion of weld ring 80 upon which metalseal 72 bears can have a carefully controlled surface to ensure that thesurface requirements of metal seal 72 are achieved. Metal seal 72 can beany suitable metal seal formed of any suitable metal or alloy. However,in some embodiments, metal seal 72 is a self-energizing metal seal, suchas a C-ring. With such seals, as the process fluid pressure increases,the sealing ability of seal 72 increases accordingly.

Once isolation diaphragm 58 is attached to weld ring 80, and diaphragmspacer 84 is attached on top of diaphragm 58, the assembly is welded tosensor body 14 at projection weld 88. Projection weld 88 projects fromweld ring 80 into sensor body 14. In accordance with embodiments of thepresent invention, projection weld 88, or any other suitable structureof weld ring 80 that bears against sensor body 14 preferably does so inalignment with metal seal 72. Thus, the force transmitted through metalseal 72 is conveyed in a substantially straight line through weld ring80 and projection weld 88. In this way, no moment or torque is createdwith process fluid pressure. Accordingly, the structure shown in FIG. 4is believed to be less susceptible to fatigue-based issues.

Isolation diaphragm 58 is typically welded to weld ring 80 as asubassembly operation. This can be done by a laser weld, in accordancewith known techniques. The subassembly weld is only for attaching thediaphragm 58 to weld ring 80 and does not directly affect the metalseal. The subassembly (weld-ring and diaphragm) is then welded to thesensor body 14 via projection weld 88. The projection weld is located indirect alignment with the seating surface of metal seal 72, with aprojection feature machined into weld ring 80. This is possible becausethe process of performing the projection weld does not affect thesurface finish of the weld ring that metal seal 72 seats against. Byhaving the metal seal 72 seated in direct alignment with projection weld88, a more equal balance of pressurized areas on weld ring 80 isachieved, resulting in increased fatigue life when subjected to processfluid pressures above 6000 psi. Additionally, by eliminating the groovethat is typically used for standard welding practices, the weld ring isactually more rigid and if necessary can employ a structurallysupporting secondary weld 86 that further increases high pressurefatigue life. Another benefit of the embodiment illustrated with respectto FIG. 4 is that the direct loading of the weld ring projection weld 88keeps a consistent hinge point of the process isolating diaphragmassembly, even under high process pressures and high flange loads.Secondary support weld 86 can be located adjacent the periphery of weldring 80 to couple weld ring 80 to sensor body 14 at a second location.Adding secondary support weld 86 provides further improvement andconsistency of the hinge point of isolation diaphragm 58. Alternativemetal seals that include greater sealing load can also be used withoutaffecting the diaphragm hinge point and they do not negatively affecttransmitter performance.

FIG. 5 is a flow diagram of a method of assembling a process fluidpressure transmitter assembly in accordance with an embodiment of thepresent invention. Method 100 begins at block 102 where an isolationdiaphragm is welded to a weld ring as a sub-assembly operation. Thisweld can be performed is accordance with any suitable techniquesincluding a laser weld. Next, at block 104, the sub-assembly is weldedto a sensor body of a process fluid pressure transmitter using aprojection weld. The location of the projection weld is selected to bealigned with a portion of the weld ring that will bear against ametallic seal. Note, blocks 102 and 104 will be performed for eachisolation sub-assembly of the process fluid pressure transmitter. Thus,if the process fluid pressure transmitter is a differential processfluid pressure transmitter, then blocks 102 and 104 will be performedtwice.

Once the isolation subassembly or subassemblies are welded to the sensorbody, the fill fluid can be added as indicated at block 106. When theprocess fluid pressure transmitter is to be mounted to a process, aflange or manifold is used to couple the process to the transmitter. Atblock 108, the process fluid pressure transmitter is mounted to amanifold using a metal seal that is substantially aligned with theprojection weld.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A process fluid pressure transmitter comprising:a sensor body including: a pressure sensor; and electronics coupled tothe pressure sensor and configured to obtain an indication of pressurefrom the pressure sensor; and at least one process fluid isolationassembly fluidically coupled to the pressure sensor and configured toreceive a process fluid, the at least one process fluid isolationassembly including an isolation diaphragm welded to a weld ring, theweld ring having a sealing surface on a first side adapted for contactwith a metal sealing ring and a weld portion welded to the sensor bodyon a second side, wherein at least a portion of the weld portion isdisposed directly over the sealing surface, such that the sealingsurface and the weld portion are substantially aligned with one another.2. The process fluid pressure transmitter of claim 1, wherein the weldportion is a projection weld portion welded to the sensor body.
 3. Theprocess fluid pressure transmitter of claim 2, wherein the at least oneprocess fluid isolation assembly includes a plurality of process fluidisolation assemblies, each being assembly being fluidically coupled tothe pressure sensor, such that the pressure sensor provides anindication of differential pressure.
 4. The process fluid pressuretransmitter of claim 2, and further comprising a spacer interposedbetween the isolation diaphragm and the sensor body.
 5. The processfluid pressure transmitter of claim 2, and further comprising a supportweld coupling the weld ring to the sensor body.
 6. The process fluidpressure transmitter of claim 5, wherein the support weld is disposedadjacent a periphery of the weld ring.
 7. The process fluid pressuretransmitter of claim 2, and further comprising a manifold coupled to theat least one process fluid isolation assembly, the manifold being scaledto the isolation assembly with a metal seal.
 8. The process fluidpressure transmitter of claim 7, wherein the metal seal is aself-energizing seal.
 9. The process fluid pressure transmitter of claim8, wherein the self-energizing seal is a c-ring.
 10. The process fluidpressure transmitter of claim 7, wherein the metal seal is substantiallyaligned with the sealing surface and the projection weld.
 11. Theprocess fluid pressure transmitter of claim 2, wherein the weld ring issubstantially rectangular in cross section.
 12. The process fluidpressure transmitter of claim 2, wherein the isolation diaphragm isfluidically coupled to the pressure sensor by a substantiallyincompressible liquid.
 13. The process fluid pressure transmitter ofclaim 2, and wherein the electronics comprises sensor circuitry coupledto the pressure sensor and transmitter circuitry coupled to the sensorcircuitry and configured to convey process fluid pressure information toanother device over a process communication link.
 14. An isolationdiaphragm subassembly for a process fluid pressure transmitter, thesubassembly comprising: a circular weld ring having a first side and asecond side, the first side having a sealing portion adapted to contacta metal sealing ring and the second side having a weld portion beingconfigured to be welded to a sensor body; an isolation diaphragm weldedto the second side of the weld ring; and wherein at least a portion ofthe weld portion is disposed directly over the sealing portion, suchthat the sealing portion and the weld portion are substantially alignedwith each other.
 15. The isolation diaphragm subassembly of claim 14,wherein the weld portion is a projection weld portion that is configuredto be projection welded to the sensor body.
 16. The isolation diaphragmassembly of claim 15, and further comprising a spacer disposed proximatethe diaphragm and a portion of the second side that is configured to beprojection welded to the sensor body.
 17. The isolation diaphragmassembly of claim 15, wherein the circular weld ring is substantiallyrectangular in cross section.
 18. A method of forming a process fluidpressure transmitter assembly, the method comprising: providing at leastone weld ring having a weldable surface and a sealing surface, oppositethe weldable surface, the sealing surface being configured to engage ametal sealing ring; welding an isolation diaphragm to the weld ring;welding the weldable surface to a sensor body of a process fluidpressure transmitter; and coupling the process fluid pressuretransmitter to a manifold using a metallic seal disposed between thesealing surface and the manifold, such that the sealing surface,metallic seal and weldable surface are substantially aligned with eachother.
 19. The method of claim 18, wherein the weldable surface is aprojection weldable surface.
 20. The method of claim 19, and furthercomprising interposing a spacer between the isolation diaphragm and thesensor body proximate the projection weldable surface.
 21. The method ofclaim 19, and further comprising providing a secondary weld between theweld ring and the sensor body.